The disadvantage being pointed out here seems to be that GTD works too well. This is not a problem with GTD. At this day and age we all invariably have 100s of projects going on. It's just that GTD makes it explicit and shows it to us.
As is being suggested, deliberately limiting the number of projects that you record in your work flow management system doesn't seem to be the solution.
Limiting the projects on purpose while relieving stress does mean those other projects that are not recorded in the task management system will creep back into your mind, will suffer inefficient execution and will weigh us down.
However the author makes a good point about how creative work isn't suitable for the GTD system. Sometimes you just don't know what you want to do. You need to be able to experiment.